The present invention relates to a method and an apparatus for heating a wood material.
Conventionally, there has been adopted a method for injecting a synthetic resin solution or various drugs into the wood material, and then heating the wood material so as to cure the injected solution therein in order to obtain a reformed wood which has improved dimensional stability, weather resistance and the like.
There have been known a method for directly heating the wood material by a hot air dryer, a hot press and the like, and a method for heating the wood material by dielectric heating means such as high frequencies or microwaves in order to heat and cure the solution injected into the wood material such that the reformed wood can be obtained.
In the case where the wood material having the injected solution is heated by heating means such as the hot air dryer or the hot press, however, there is caused a problem that cracks, warps, deflections, kinks, collapses and the like occur when the injected solution is cured. In addition, when a solvent is volatilized, the injected solution flows out of the wood material so that the residual rate of the injected solution is reduced. Consequently, reforming effects are lowered, and the curing defects of the injected solution are caused. In the case where a thick wood material is subjected to a heat treatment, particularly, the above-mentioned problems become more remarkable.
In the case where the injected solution is cured, the cracks and the like are caused for the following reasons. When the wood material is heated, the solvent in the wood material is rapidly transferred. Consequently, there is increased the difference of solvent content between the central portion and the surface layer portion of the wood material. Thus, there is caused the difference of shrinkage stress between the central portion and the surface layer portion of the wood material, or the irregular shrinkage of the wood material. In addition, the synthetic resin solution in the surface layer portion is cured more quickly than in the central portion, so that the solvent remaining in the central portion is prevented from being discharged out of the wood material. Consequently, explosions are caused by the steam pressure in the central portion of the wood material.
On the other hand, when the wood material is heated by the dielectric heating means such as high frequencies or microwaves, the wood material is dielectrically heated from its inside. Consequently, the injected solution is also cured from the inside of the wood material. Thus, the above-mentioned problems are eliminated. In the case where only the dielectric heating means is used, however, a temperature tends to be increased and decreased in the vicinity of plus and minus electrode plates, respectively. As a result, there is caused the unevenness of temperature owing to the arrangement of the electrode plates. Consequently, curing defective portions are generated. In addition, there is increased the density of the solvent as a component of the injected solution.
The present inventors have proposed that external heating means such as a heating plate having a heating wire is used together with the dielectric heating means (see Japanese Patent Application No. 326443/89). However, there are caused the following problems. More specifically, a lead wire and the heating wire of the heating plate generate heat owing to high frequencies, so that the heating plate is damaged. In addition, the influence of the high frequencies is transmitted to other equipments through the lead wire.
In the case where the solution containing a combustible solvent is to be injected into the thick wood material, a heating temperature for curing the injected solution in the wood material should be increased because of the great thickness of the wood material. Consequently, the solvent may cause fires or explosions.
In the cases where the wood material is heated to cure the injected solution therein and where the wood material is heated to evaporate water therein such that it is dried, the above-mentioned problems are caused.